Television camera tube

ABSTRACT

A television camera tube (vidicon) having a collimation lens for directing the deflected electron beam at right angles to the photoconductive layer. The collimation lens comprises no gauzeshaped electrode. A special configuration is described which results in small lens aberrations and a small landing error together with a large field strength at the surface of the photoconductive layer. The configuration is also such that the so-called ion spot is prevented.

United States Patent Gerlach Apr. 2, 1974 [54] TELEVISION CAMERA TUBE 3,023,336 2/1962 Frenkel 313/78 1 Invent Georg Germ Emmasingel, 3123313331 3132? iliififfiiffiii: .11131354'2 Emdhoven Netherlands 3,040,205 6/1962 Walker 315 14 x [73] Assigneez Us. Philips Corporation New 3,461,340 8/1969 Charles et al. 313/68 R X York, NY. Primary Examiner-Carl D. Quarforth [22] led: 1972 Assistant Examiner-P. A. Nelson [21] Appl. No.: 298,622 Attorney, Agent, or Firm-Frank R. Trifari [30] Foreign Application Priority Data [57] W 7 ABSTRACT Nov. 6, 1971 Netherlands 7115320 A television Camera tube having a collimation lens for directing the deflected electron beam at 521 11s. (:1 315/14, 315/15, 313/78 right angles to the Photoconductive y The colli- 51 Int. Cl. 11013 29/56 mation lens comprises no gauze-Shaped electrode A 53 Field f Search 315/14 15; 313/ 7 3 R, special configuration is described which results in 313/76 77 73 small lens aberrations and a small landing error together with a large field strength at the surface of the [56] R f r n Ci photoconductive layer. The configuration is also such Pietri 315/15 X that the so-called ion spot is prevented.

2 Claims, 1 Drawing Figure TELEVISION CAMERA TUBE The invention relates to a television camera tube comprising an electron gun for producing an electron beam and a flat photoconductive layer which is provided on a transparent conductive signal layer, on the surface of which photoconductive layer which is not in contact with the signal layer a potential distribution is formed by projecting an optical image on the photoconductive layer, said potential distribution being periodically reduced to substantially the potential of the cathode of the electron gun by scanning the photoconductive layer with the electron beam, said television camera tube furthermore comprising a number of rotati onally symmetric electrodes'for focusing the electron beam onto the signal layer, deflection means for deflecting the electron beam from an effective deflection point, and a collimation lens for directing the deflected electron beam at right angles to the photoconductive layer.

Such a television camera tube is shown and is termed Vidicon. The operation of a vidicon is as follows. Under the influence of the deflection means an electron beam of a sufficient current strength scans the free surface of the photoconductive layer according to a given frame and brings said surface point-wise at the potential of the cathode which is termed zero volt. Between two successive scans, the potential of each point of the free surface of the photoconductive layer increases under the influence of a positive potential which is applied to the signal layer and under the influence of photoconductivity which is produced in the photoconductive layer by the optical image projected thereon. Each point, or more exactly each elementary surface element, of the free surface of the photoconductive layer together with the signal layer constitutes a capacitor. The charge of said capacitor which decreases as a result of the photoconductivity is periodically replenished by the scanning electron beam for which purpose more charge is necessary as more light impinges upon the relevant point. The signal current which consequently flows through the connection of the signal layer contains the information of the projected optical image as a function of time.

The current strength of the electron beam must be sufficiently large to provide sufficient change to elementary capacitors, which as a result of large light intensity are strongly discharged. As soon as the free surface of the photosensitive layer has reduced to zero volt in a given point, the electrons of the electron beam can just no longer reach said point. It is assumed that the electrons approach the surface of the photoconductive layer at right angles. Electrons which do not approach the photoconductive layer at right angles have too small a velocity component at right angles to the photoconductive layer to be able to reach a place having a potential of zero volt. An electron beam which is incident at an angle thus does not reduce the potential of the photoconductive layer to zero volt but to a given positive potential the value of which depends upon the angle at which the electron beam approaches the photoconductive layer. Said potential is termed landing error. In practice, the landing error must be as small as possible because otherwise the charge which is necessary for erasing the potential distribution on the photoconductive layer and which forms the signal current is dependent on the landing error and does not correctly reproduce the picture information. The landing error also influences the velocity distribution of the electrons which can reach the photoconductive layer. Said velocity distribution which is inter alia determined by the spread in the velocities with which the electrons leave the cathode, is one of the factors which influence the response velocity, i.e. the velocity at which the camera tube reacts to light variations. The influence of the landing error on the response velocity is larger as the applied photoconductive layer itself has a smaller inertia and hence the camera tube a larger response velocity.

In order to direct the electron beam at right angles to the photoconductive layer, a vidicon comprises a collimation lens. The collimation lens is an electron optical lens a focus of which coincides with the effective deflection point of the deflection means, as a result of which the emerging electron beam extends parallel to the axis of the lens. Effective deflection point is to be understood to mean the point of intersection of the tangents at the axis of the not yet deflected and the deflected electron beam. Since the collimation lens also influences the focusing of the electron beam on the photoconductive layer, the proportioning of such a lens generally is a compromise. Up till now, the collimation lens, if at least the highest requirements as regards the landing error and lens aberrations are to be fulfilled, is inter alia formed by a gauze-like electrode parallel to and immediately in front of the photoconductive layer and at a high positive potential. The gauze of said electrode must be extremely fine since the diameter of the spot of the electron beam is only a few tens of am. This gauze causes many problems in manufacturing the vidicon in particular as a result of small defects in the gauze or dust which falls on it during assembly. Nevertheless, the gauze-shaped electrode was thus far considered to be unavoidable, the more so since said electrode has to fulfill another two functions. First of all, the field strength on the photoconductive layer is large as a result of the large potential difference and the small distance between the gauze and the photoconductive layer. This is absolutely necessary because the electrons have to be braked to approximately zero volt over a very small distance. The track of electrons which would have only a small velocity while they are still at a comparatively large distance from the photoconductive layer, is actually influenced by the potentials of the photoconductive layer in the vicinity of the point where the beam should impinge. This means that it is difficult for the electron beam to reach places on the photoconductive layer which are only little discharged if there are places in the vicinity which are strongly discharged. As a matter of fact the strongly discharged places are strongly positive and attract the electron beam. This has for its result that bright parts of the projected image swell up in the displayed television picture at the expense of darker parts. This phenomenon is worse as the field strength on the photoconductive layer is smaller. Secondly, the gauze-shaped electrode serves to retain positive ions which are formed from gas residues in the tube as a result of ionisation by the electron beam. Actually, the gauze-shaped electrode generally has the highest positive potential in the tube and can thus not be reached by the positive ions which are formed before the gauze-shaped electrode. Said positive ions can as a result not reach the photoconductive layer either, as a result of which no so-called ion spot is formed in the displayed picture as a result of signal current produced by said ions. The positive ions formed behind the gauzed-shaped electrode are harmless because they are regularly scattered over the whole photoconductive layer.

A vidicon without a gauze-shaped electrode is known from the U.S. Pat. No. 3,040,205. The collimation lens described in said Pat. consists of a number of annular electrodes with decreasing potential in the direction towards the photoconductive layer. A drawback of this collimation lens is that many electrodes are required. Although the patent specification states that a number smaller than the six rings shown will also be sufficient, no doubt lens aberrations and landing errors will result if an accurately described configuration is not used. Obviously, less high requirements as regards the landing error are imposed in the tube described in the said patent specification since said tube has two deflection points for the deflection directions mutually at right angles, as a result of which the focus of the collimation lens has to be laid between the said two deflection points and a landing error remains for the two directions of deflection.

It is the object of the invention to provide a vidicon which comprises a collimation lens without a gauzeshaped electrode and having a minimum of other electrodes, which collimation lens has very small lens aberrations and causes very small landing errors. According to the invention, a television camera tube of the type described in the preamble is characterized in that, taken from the photoconductive layer, the collimation lens is formed by first and second rotationally symmetric electrodes and the photoconductive layer, that the smallest inside diameter of the first rotationally symmetric electrode is equal to 1.2 i percent times the smallest inside diameter of the second rotationally symmetric electrode, that the effective length of the first rotationally symmetric electrode is equal to 0.75 i 20 percent times the smallest inside diameter of the first rotationally symmetric electrode, and that the television camera tube comprises means for supplying to the second rotationally symmetric electrode a positive potential which is highest calculated relative to the potential of the cathode, and means for supplying a potential to the first rotationally symmetric electrode which is lower than 0.2 times the potential of the second rotationally symmetric electrode. Extensive calculations and investigations in a large number of possible configurations for the collimation lens have proved that with said configuration both small lens aberrations and landing errors are formed and a large field strength at the surface of the photoconductive layer. On the side of the photoconductive layer, the configuration has only one rotationally symmetric electrode having a potential which is lower than the highest potential in the tube.

A television camera tube according to the invention furthermore preferably comprises a third rotationally symmetric electrode which, taken from the photoconductive layer, immediately succeeds the second rotationally symmetric electrode, and means for supplying to the third rotationally symmetric electrode a potential which is equal to 0.7 i 20 percent times the potential of the second rotationally symmetric electrode. As a result of this, the formation of the already mentioned ion spot is prevented because the positive ions which are formed in front of the second rotationally symmetric electrode cannot pass said electrode since said electrode has the highest positive potential in the tube and the positive ions which are formed behind said electrode are regularly scattered throughout the photoconductive layer.

The invention will be described in greater detail, by way of example, with reference to the accompanying drawing which shows one of the possible embodiments of a vidicon according to the invention.

The television camera tube shown is of the type Plumbicon (registered trade mark) and comprises a photoconductive layer 1 of lead monoxide which is provided on a transparent conductive signal layer 2 of tin oxide. Such a photoconductive layer itself reacts with a comparatively small inertia to light variations. As a result of this, it is very important just for this type of tube, that the landing error be small. The tube comprises a glass envelope 3 and an electron gun 4 having a cathode 5, a first grid 6 and a second grid 7. The tube furthermore comprises a focusing electrode 8, a collector electrode 9, an ion trap electrode 10 and a collimation electrode 11. The electrodes 8, 9 and 10 are metal sleeves. The electrode 11 consists of a conductive layer on the inner wall of the envelope 3. The tube furthermore comprises connection pins 12 and a set of deflection coils 13 which deflect the electron beam 16 produced by the electron gun 4. The effective deflection point of the deflection coils 13 is denoted by 14. The distance from the deflection point 14 to the photoconductive layer 1 is denoted by A. The inside diameter of the ion trap electrode 10 is denoted by B and of the collimation electrode 11 by C. The effective length of the collimation electrode 11 is denoted by D and is calculated from the photoconductive layer 1 to the boundary plane 15 of the ion trap electrode 10. A collimation electrode which is divided into two or more parts which have the same or substantially the same potential is considered to be one electrode within the scope of the invention.

The embodiment shown has the following specifications:

Dimensions: A 18 mm B=2l mm C=23 mm D=15 mm.

The length of the diagonal of the scanned part of the photoconductive layer is 17 mm.

Voltgge sg gatlggdej 0 volt first grid 6 30 volt second grid 7 300 volt focusing electrode 8 I000 volt collector electrode 9 4000 volt ion trap electrode 10 6900 volt collimation electrode 1 l 450 volt signal layer 2 40 volt The collimation lens which directs the deflected electron beam at right angles to the photoconductive layer 1 consists of the ion trap electrode 110, the collimation electrode 11, and the photoconductive layer 1. In the case of an accurate perpendicular landing, electrons of the electron beam which have left the cathode at velocity zero, can reach a potential which is equal to the cathode potential (0 volt). The deviation from the perpendicular landing of the electron beam is indicated by the landing error, which is the lowest potential at the photoconductive layer which electrons of the electron beam which have left the cathode at velocity zero can still reach. The landing error achieved with the configuration shown varies of course over the surface of the photoconductive layer l and is maximum 1 volt. The lens aberrations are such that focusing lens (electrodes 7, 8 and 9) and the collimation lens (electrodes 10, 11 and 1) together give a spherical aberration of 5 urn. The coma is 2 pm, the astigmatism 48 un and the distortion 16 percent. The distortion is reduced in known manner, to an acceptable value of, for example, 5 percent, by a suitable shape of the deflection coils 13, by superimposing a correction voltage which depends on the deflection on the direct voltage which the collector electrode 9 conveys, or by superimposing a correction current on the current through the deflection coils 13 and by a combination of a few of these measures, respectively. The field strength at the surface of the photoconductive layer is minimum 150 V/mm which is above the required minimum.

What is claimed is: 1. A television camera tube comprising: a. an evacuated envelope having a transparent wall portion; b. a transparent conductive signal layer disposed in said envelope at said wall portion; c. a photoconductive target layer disposed on said signal layer and facing said electron gun means; d. electron gun means within said envelope for directing electron beam to said target layer, said gun means comprising a cathode; e. rotationally symmetric electrode means for focusing said electron beam onto said photoconductive target layer;

f. deflection means for deflecting said electron beam from an effective deflection point;

g. collimation lens means for directing a deflected said electron beam at substantially right angles to said target layer said collimation lens means comprising first and second rotationally symmetric electrodes and said photoconductive target layer, said second electrode being spaced from said target layer and said first layer being disposed therebetween, said second electrode having a certain smallest inside diamter and said first electrode having a smallest inside diameter substantially equal to 1.2 i 0.24 times said certain smallest inside diameter, said first electrode having an effective length substantially equal to 0.75 :t 0.15 times said smallest inside diameter of said first electrode;

h. means for applying to said second electrode a first positive potential, said first positive potential being the highest tube potential relative to the potential of said cathode; and

i. means for applying to said first electrode a potential below about 0.2 times said first potential.

2. A television camera tube as recited in claim 1, wherein said television camera tube comprises a third rotationally symmetric electrode spaced from said target layer with said first and second electrodes disposed therebetween and means for supplying to the third rotationally symmetric electrode a potential which is substantially equal to 0.7 i 0.14 times said first positive potential. 

1. A television camera tube comprising: a. an evacuated envelope having a transparent wall portion; b. a transparent conductive signal layer disposed in said envelope at said wall portion; c. a photoconductive target layer disposed on said signal layer and facing said electron gun means; d. electron gun means within said envelope for directing electron beam to said target layer, said gun means comprising a cathode; e. rotationally symmetric electrode means for focusing said electron beam onto said photoconductive target layer; f. deflection means for deflecting said electron beam from an effective deflection point; g. collimation lens means for directing a deflected said electron beam at substantially right angles to said target layer said collimation lens means comprising first and second rotationally symmetric electrodes and said photoconductive target layer, said second electrode being spaced from said target layer and said first layer being disposed therebetween, said second electrode having a certain smallest inside diamter and said first electrode having a smallest inside diameter substantially equal to 1.2 + OR - 0.24 times said certain smallest inside diameter, said first electrode having an effective length substantially equal to 0.75 + OR - 0.15 times said smallest inside diameter of said first electrode; h. means for applying to said second electrode a first positive potential, said first positive potential being the highest tube potential relative to the potential of said cathode; and i. means for applying to said first electrode a potential below about 0.2 times said first potential.
 2. A television camera tube as recited in claim 1, wherein said television camera tube comprises a third rotationally symmetric electrode spaced from said target layer with said first and second electrodes disposed therebetween and means for supplying to the third rotationally symmetric electrode a potential which is substantially equal to 0.7 + or - 0.14 times said first positive potential. 